Faulty Taptic Engines may be behind the extremely limited availability of the Apple Watch, The Wall Street Journal reported on Wednesday. Citing sources familiar with the matter, the publication says that after mass production of the Engines began in February, quality testing found some of them to be unreliable.

The component, which Apple uses in its Watch to produce the sensation of being tapped on the wrist, is made by two suppliers: AAC Technologies Holdings Inc. and Nidec Corp. Apparently some of AAC’s Taptic Engines were found to break down overtime, so Apple has moved a majority of its production over to Nidec.

It’s not clear how much the Taptic Engine issues have contributed to the Apple Watch shortages, but The Journal says that Apple told some Watch suppliers last week to slow production until June, without explaining why. If true, the move would be questionable considering how far below Watch supply is from demand.

Apple has faced considerable criticism for the way it has handled the Apple Watch launch thus far. The company, once well known for its supply chain savvy and ability to rollout products quickly, has only shipped a small fraction of pre-orders thus far, and it’ll likely be June before retail stores get any real inventory.